Messages that target all current members are sent as multicast messages. However, these retransmissions can be replayed on new nodes that have just joined the cluster. This can result for example in execution of already completed transaction on the new node, causing possible data inconsistency for those entries which are owned by the new node in backup way - the replayed transaction sequence authoritatively overwrites them. The node should remember the first topologyId it has seen and do not execute any commands that have lower topologyId.
Dan Berindei <dberinde> made a comment on jira ISPN-3731 Before my fix for ISPN-3670, commands received before the initial topology used to be ignored. Of course, this caused other problems, because some commands shouldn't be ignored (the ones that were sent explicitly to the joiner, because some nodes already see it as a member of the write CH).